The present invention relates to an adsorption purification unit comprising the combination of a regenerative component comprising structured adsorbents and of an adsorber filled with a particulate adsorbent.
When gases are to be produced, separated or purified, use may be made of adsorption processes. Use is generally made of several adsorbers filled with adsorbent materials that are selective with respect to at least one of the constituents of the feed stream. There are two main adsorber technologies, one being axial bed adsorbers and the other radial bed adsorbers. In the first case, the gas circulates vertically through an adsorbent bed, in the second case the gas circulates radially, either from the inside toward the outside (relative to the adsorption phase) in centrifugal configuration, or from the outside towards the inside in centripetal configuration.
Within the context of the invention, mention will respectively be made of PSA (pressure swing adsorption), VSA (vacuum swing adsorption) and (V)PSA denoting one or other of the 2 units but also a combination of the 2.
The axial technology is not very expensive but when high flow rates are treated the pressure drops and the problems of attrition become limiting. Thus, starting from a certain flow rate to be treated, one solution consists in changing to radial geometry that results in a limitation of the pressure drops without an increase in the radius of the adsorber. Specifically, the radial adsorber offers an increased flow area for a given volume of adsorber and is not theoretically subject to a limitation with respect to the attrition phenomena. The bed of adsorbent may be suspended between vertical perforated grids suspended by the top. The best-known drawbacks of this radial technology are an increase in the dead volumes and a high manufacturing cost.
Nevertheless, another drawback linked to this radial technology appears when one of the beds is of smaller size compared to the others.
For example, a PSA or TSA adsorption process will be considered comprising two types of adsorbents (A and B) requiring passage of the gas through A before B in the adsorption phase and for which the amount of adsorbent B needed is very large relative to the amount A. Since the adsorber comprises 2 beds, 3 grids are generally used to hold the particulate materials. In centripetal radial configuration, the material A is located between the “outer” grid and the “intermediate” grid whereas the material B is held between the same intermediate grid and the “inner” grid. This A/B disproportion then accentuates, on the one hand, the difficulties in constructing said radial adsorber since the diameters of the outer and intermediate grids are similar, and consequently makes it difficult to maintain a uniform thickness of the bed due to non-ideal characteristics and possible deformations of the grids that could lead to preferential pathways in the zones where the screen thickness is less.
To overcome these drawbacks, one solution consists in reversing the flow direction of the gases and also the distribution of the adsorbents, so that the adsorbent A is between the inner grid and the intermediate grid and the adsorbent B is between the intermediate grid and the outer grid. With a flow of the gas from the inside toward the outside of the vessel in the adsorption phase, the adsorber is therefore in “centrifugal radial” configuration (FIG. 1).
However, this centrifugal configuration may prove less energy-efficient than the centripetal solution. Mention will be made, for example, of the case of the O2 VSA process where this centrifugal configuration substantially increases the pressure drops and is consequently detrimental to the specific energy of the process, and also the case of TSA processes where the regeneration from the outside to the inside will increase the heat losses.
O2 VSA processes conventionally consist of two beds, the first being a low-volume layer of alumina (silica gel or certain zeolites are also used alone or in combination), the objective of which is to stop the water contained in the feed air and the second is a zeolite layer that selectively retains nitrogen with respect to oxygen.
One geometry that makes it possible to retain the centripetal configuration, referred to as the “mushroom” configuration, has been used for these O2 VSA processes. It consisted in installing in the bottom of the adsorber a layer of granulated alumina held between two grids with a radial circulation of the fluid, or more simply positioned in axial configuration. Although this solution makes it possible to retain a centripetal radial configuration for the zeolite, it nevertheless substantially complicates the construction and leads to a significant additional cost.
Also added to these hydrodynamic problems are drawbacks linked to the presence of several selective adsorbents. To mention the case of the O2 VSA process, the use of alumina in granular form, which as described above has the role of dehumidifying the gas to be treated, today limits the performance levels, in particular the specific energy and the productivity, of such processes. Specifically, the addition of an alumina layer to the adsorber substantially increases the dead volumes and also the pressure drops. Lastly, alumina, due to its physical properties, acts as a thermal insulator/accumulator leading to the storage of frigories at the interface with the screen, a phenomenon that is substantially detrimental to the specific energy of the system. Decoupling the alumina from one or more other adsorbents used would thus make it possible to benefit from significant savings in the pumping energy.
Starting from there, one problem that is faced is to provide a novel configuration that makes it possible to overcome all these drawbacks.